1. Field of the Invention
The present invention relates to processes of low-level mixed waste solidification.
2. Related Art
Low-level mixed waste contains hazardous chemical and low-level radioactive species. The chemical contaminants are often volatile compounds or pyrophorics and cannot be disposed of by conventional high-temperature methods.
Portland cement grouting (PCG) is one of the prior art radioactive waste solidification methods. PCG results in hydration-induced hardening. In addition to hydration-induced hardening cements, there are also chemical hardening cements. Various phosphate compositions belong to the class of chemical hardening cements. The solidification of phosphate compositions results from a number of chemical reactions of metal oxides and orthophosphoric acid at room temperature, thereby causing generation of a hard solid phosphate form with a low solubility in water. These phosphate forms are very efficient for immobilization of rare earth and transuranic elements.
Radioactive and toxic incinerator ash has also been immobilized by incorporating the ash into cements based on zirconium orthophosphate and dual magnesium-sodium and magnesium-ammonia orthophosphate. Immobilization in chemical hardening cements is caused by both physical isolation of the dispersed hazardous elements and their structural integration into the phosphate matrix upon its formation.
Phosphate binders are heterogenous systems consisting of a powder with basic properties (metal oxide or hydroxide) and phosphoric acid. Chemical reactions between the two leads to self-setting of such a system. The products of this reaction are hydrated salts of orthophosphoric acid that can be characterized as inorganic polymers. These polymers, also defined in some literature as phosphate ceramics, have several desirable characteristics including: high compression strength, adhesion to inert surfaces, insolubility in water and the ability to withstand very high temperatures.
The relevant prior art processes concern incinerator ash immobilization in a magnesium-phosphate cement matrix. The prior art process is implemented by the following operations:
1. Preparation of cement powder by mixing magnesium oxide powders calcined at 1,000.degree. C., and 15 mass % boric acid (reaction moderator); PA1 1. Longer hardening period, while the hardening moderator, i.e. iron oxide (3+), also functions as the matrix material; PA1 2. Low cost of the input materials, i.e. iron oxides; PA1 3. Metallurgical waste (cinder) and iron-containing natural minerals (magnetite) are used as the input matrix material; PA1 4. Ferro-magnetic properties of the matrix provide for remote transfer of the radiation hazardous compounds by means of the electromagnetic equipment; and PA1 5. Capability to control the setting time by varying the concentration of Fe.sub.2 O.sub.3 in the system. PA1 (a) forming a mixture of iron oxide powder having ratios in mass % of FeO:Fe.sub.2 O.sub.3 :Fe.sub.3 O.sub.4 =25-40:40-10:35-50; PA1 (b) forming a powder phase of waste powder and the mixture of iron oxide powder; PA1 (c) forming a solution of orthophosphoric acid H.sub.3 PO.sub.4 ; PA1 (d) mixing the solution of orthophosphoric acid with the powder phase of waste powder and the mixture of iron oxide powder in mass % of waste powder:iron oxide powder:acid solution=30-60:15-10:55-30 to form a slurry; PA1 (e) blending the slurry to form a homogeneous mixture; PA1 (f) setting the slurry to form monolithic specimens; and PA1 (g) curing the homogeneous mixture at room temperature to form a final product. PA1 (a) preparing a predetermined amount of magnetite powder; PA1 (b) forming a powder phase of waste powder and the magnetite powder; PA1 (c) forming a solution of orthophosphoric acid and ferric oxide; PA1 (d) mixing the solution of orthophosphoric acid and ferric oxide with the powder phase of waste powder and magnetite in mass % of waste powder:magnetite powder:acid solution=30-60:15-10:55-30 to form a slurry; PA1 (e) blending the slurry to form a homogeneous mixture; and PA1 (f) curing the homogeneous mixture at room temperature to form a final product.
2. Mixing of the generated cement powder and the ash powder;
3. Mixing of the generated powder and 50% orthophosphoric acid solution; and
4. Molding and setting of the samples.
The generated magnesium-phosphate cement samples incorporate 35 mass % of the incinerator ash. The compression strength of these samples is 275 kg/cm.sub.2. The leach rate data for toxic and radioactive metals have been obtained by using Environmental Protection Agency Method 1311, Toxicity Characteristic Leaching Procedure (TCLP) and American Nuclear Society, American National Standards Institute Measurement of the Leachability in Solidified Low-Level Radioactive Wastes by a Short-Term Test Procedure, Method (ANSI 16.1). The leach rate values for the toxic metals obtained by TCLP method do not exceed the established limits, while the determined leachability indices for various elements, estimated by ANSI 16.1, range from 15 to 22, thereby exceeding the passing criterion of 6 set by the Nuclear Regulatory Commission (NRC).
The sample mass loss during long-lasting leaching tests does not exceed 1%. Therefore, the characteristics of the magnesium-phosphate materials that incorporate incinerator ash indicate their high chemical stability and compression strength.
The prior art magnesium-phosphate cement used for incinerator ash immobilization suffers from a number of disadvantages. Magnesium oxide is expensive and requires high-temperature annealing to slow down its reaction speed. In addition, the composition is difficult to mix because of how fast it hardens. The prior art magnesium-phosphate cements also require the application of additives that function as moderators, e.g. boric acid, to slow down hardening.